Luminaires of the type commonly used for street lighting comprise an elongated housing containing a lamp, a reflector and a refractor at the front end, and a mounting device known as the slipfitter at the rear end. The slipfitter receives and clamps a generally horizontal pipe support in a manner allowing adjustable mounting of the luminaire. General desiderata in slipfitter design are adaptability to various pipe sizes, adjustability with a minimum number of parts, ease of assembly at the factory and convenience in installation and adjustment in the field, combined of course with low cost.